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Sedimentary Rocks

•When rocks are broken down into fragments, either through the mechanical means ofweathering, or through chemical reactions, the fragments are called sediment. When thatsediment is compacted or cemented together, it forms a sedimentaryrock.

•Sedimentary rocks are the most common rocks that we encounter, because the Earth hasso many efficient weathering processes that constantly break down rock and createsediment.

•Common sedimentary rocks include sandstone (made up, not surprisingly, of sand) andlimestone (made up of calcium carbonate precipitated out of solution).

•Sediments become sedimentary rock through compaction (squeezing sediments togetherand forcing out any fluids) and cementation (introduction of a cementing agent).

•Sediments are either clastic or chemical. That is, rocks are broken down through eithermechanical or chemical means.

Clastic sediment

•Clastic sediment is what one usually thinks of when speaking of sediment. From theGreek word klastos (broken), it refers to the broken remains of rocks of all types, brokenand altered by weathering processes such as wind, water and ice. Clastic sediment is alsoknown as detrital sediment.

•Clastic rocks (sedimentary) differ from igneous rocks not by the mineral compositionbut by the grain texture that shows its history of mechanical weathering, the cementationmaterial and the possible presence of fossils (which cannot survive the high temperaturesrequired to melt igneous rocks).

•Sedimentary layering is also a powerful clue to sedimentary origin. Igneous rocks haveminerals which grow in every direction. Sedimentary rocks, on the other hand, are oftenlaid down in layers that mimic the horizontal surface of the Earth.

Chemical sediment

•Chemical sedimentary rocks may

contain fossils and other sedimentary characteristics,but their components were not broken up mechanically. Rather, rocks were dissolved insolution (as salt can dissolve in water) and transported, then precipitated chemically (assalt can precipitate out of a saturated solution).

•Precipitation can occur biochemically. Example: tiny plants living in seawater candecrease the acidity of the surrounding water and so cause calcium carbonate toprecipitate.

•Precipitation can also occur through inorganicmeans. Example: As seawaterevaporates, it often leaves behind salts which have precipitated out. This is how halite(salt) is formed.

Lithification of a sedimentary rock

•Once either clastic or chemical sediment is formed, it becomes sedimentary rock throughsediment transport, deposition and diagenesis, or the alteration of sediments that createrock out of deposited sediments.

•Each of these steps occur in many ways, but each of these processes will leave clues inthe characteristics of the rocks.

Transport

•Sediment can be transported in many ways. The main types of transport are throughwind action, water action, glacial or ice action, or mass wasting (mass movement ofEarth, through avalanches, slides, slumps and so on).

•Each of these processes leaves telltale marks on the rocks that are produced.

•Example: Sediments transported by water often have rounded, sorted grains because ofthe abrading, polishing action of the sediments transported in the flow.

•Example: Sediments transported by glaciers are often broken and shattered by thegrinding action of the travelling glacier. Thus, the sediments are commonly angular ratherthan rounded.

Deposition

•Sediment deposition occurs when the method of transport is unable to carry the sedimentgrains.

•This is related to the size and weight of the grains and thus is an indicator of the powerand speed of the transporting mechanism.

•Example: Wind action cannot transport grains much larger than sand.

•Example: Glaciers are slow movers, but becausethey are so large, they can carrysediment the size of boulders for great distances, depositing them only when the ice meltsand retreats.

Wind-transported sediment is usually no larger than sand-sized.

Glacial till can be comprised of all sizes of

sediment from silt and mud to boulders.

Diagenesis or Lithification

•Changes that sediment undergoes after deposition are referred to as diagenesis. Thisincludes any transformations during and after formation into a rock. The actual process ofrock formation from sediment is called lithification.

•Diagenesis occurs through the processes of compaction, cementation, recrystallizationand chemical alterations of the sediment.

•Compaction is the simplest change that sediments can undergo.

•Compaction occurs as the weight of accumulating sediment forces the rock and mineralgrains together. This reduces pore space and eliminates some of the contained water.

•Cementation is a result of water circulating through the pore spaces of a sediment.

•If that water carries in it dissolved substances that then precipitate out during circulation,those substances left behind in the sediments can act as cementing agents.

•Calcium carbonate is one of the most common cements because it is found abundantly inseawater.

•After burial, occasionally less stable minerals may change to more stable forms throughrecrystallization. That is, the minerals begin to grow again, interlocking with each other.

•This process is important in the alteration of more porous limestone to harder, morecompact versions of this mineral.

•Chemical alterations can also affect sediments.

•If oxygen is present, organic remains are quickly converted into carbon dioxide andwater. This is called an oxydizing environment.

•In the absence of

oxygen to bond with, organics will be transformed into solid carbon,such as peat and coal.

•From a planetary perspective, chemical alterations can also occur. For example, thewind-transported sediments of Mars have been subjected to the chemical process of rust.

Classifying sedimentary rocks

•Sediments can be distinguished from other rock types by the presence of stratification,by the size and texture of the grains, and by the presence of grain sorting.

•Although each of these characteristics may not

be present, those that are, are usuallyexcellent clues as to the processes that formed the sedimentary rocks in question.

Stratification (layering)

•Because sediments are typically laid down at the Earth’s surface, they tend to form inhorizontal layers. These are called strata (singular = stratum).

•This layered arrangement of strata in a body of rock is referred to as bedding. Each bedmay be different from those around it, in thickness, in typical grain size, or in othercharacteristics.

•The differences in beds are clues to changes in the way sediments were deposited ortransported.

Cross-bedded standstone near Kanab, Utah, consists of ancient sand dunes that have beenconverted to sedimentary rock. Cross strata are inclined to the right, in the

directiontoward which the winds were blowing.

•Planar, or parallel bedding usually occurs in water, where currents or waves areminimal.

•Ripple marked bedding indicates wave or wind action.

•Cross bedding are the work of turbulent flow in water or

wind (dunes).

•Graded bedding, or sorting of grains with large ones near the bottom, is usually formedfrom standing water in which sediments have been able to settle out.

•Example: Wapatki sandstone, with grains almost too small to be seen. These sedimentswere laid down by slow-moving water.

Grain texture

•Grain texture refers to the angularity or roundedness of the grains in a sedimentary rock.

•Angular grains are commonly broken from bedrock, while rounded grains have beenabraded by other fragments.

•Typically the more rounded a grain is, the more time that grain has come into contactwith other grains, and thus the longer the distance of travel for that grain.

Sorting

•Sorting is a conspicuous feature of many sedimentary rocks. Sorting is the process bywhich grains of like size or weight are grouped together in a rock. This process is mostcommon in air or water transported sediments.

•Sorting is often an indicator of the energy of the transport mechanism: poor sortingmeans lower energy or glacial transport.

•Grains can be sorted by specific gravity, for instance, as heavier particles carried in astream may fall out of the flow before lighter particles.

•Grains in a rock or bed may also be sorted by size, which is more important when mostgrains are of the same mineral (and thus similar specific gravity).

•Finally, if transport if sediment is particularly long, grains may be

sorted by durability.Thus, in some cases, the only grains that survive the distance are the most resistant toweathering and fracture—

typically quartz.

Well sorted grains in a sandstone sample.

Poorly sorted grains in a conglomerate.

Other features

•Other features that are found commonly in sedimentary rocks include fossils and varioussurface features.

•Fossils, the remnants of animals and plants, are often buried with sediments, protectedagainst oxydation and erosion, and are converted to rock. Fossils are indicators of theclimate, surface or sea temperature, and precipitation at the time of burial.

•Ripple marks from waves and mud cracks of dry lake beds can also be preserved andlithified to create sedimentary rock.

Sedimentary environments

•Sediments (and thus sedimentary rocks) are formed in many environments, which can begrouped into four large categories: non-marine water environments, ice or glacialenvironments, eolian environments and ocean environments.

•Non-marine water sediments are formed in streams and lakes. Both mechanical andchemical sediments are important. Lakes or dry lake beds, especially, are places to lookfor chemically deposited sediment.

•Sediments eroded and transported by glaciers are deposited at the base of the glacier orreleased at its edge during melting.

•Wind-transported sediments are typically found in eolian environments, better known asdeserts.

•Ocean sediments are formed either in near-shore environments such as beaches, lagoonsand deltas, offshore on the continental shelf (like reefs), or in the deep sea.